GABAergic interneurons are the primary source of inhibition in the nervous system and play critical roles in every brain circuit. The abnormal development and function of cortical interneurons has been implicated in the pathobiology of many neurological disorders such as schizophrenia, autism, and epilepsy. As the onset of these diseases presents prior to adulthood, a better understanding of interneuron differentiation and maturation in normal development and disease models is required. This K99/R00 application will support Dr. Timothy Petros in his pursuit of developing innovative strategies and acquiring new skill sets to study interneuron development. The experiments will be initiated during the mentored period (carried out in Dr. Gord Fishell's lab at NYU) and continue in Dr. Petros' own laboratory upon securing an independent position. Dr. Petros' long-term career goal is to unravel the mechanisms that direct interneuron fate determination and maturation, with the hope of developing new therapeutic targets for treating a variety of neurological diseases. Dr. Petros' previous work has identified a novel mechanism regulating fate decisions for the largest, clinically- relevant interneuron populations arising from the medial ganglionic eminence (MGE): parvalbumin-expressing (PV+) cells preferentially arise from intermediate neuronal progenitors (INPs) via indirect neurogenesis whereas direct neurogenesis primarily gives rise to somatostatin-expressing (Sst+) interneurons. The molecular mechanisms that regulate this decision remain unknown, but out preliminary data demonstrate a strong potential for involvement of Jagged1 and notch signaling in this process. Specific Aim 1 will utilize multiple in vitro and in vivo approaches to investigate the hypothesis that Jagged1 inhibits notch activity in the MGE to promote direct neurogenesis and formation of Sst+ cells. The MGE is a heterogeneous population of progenitors that gives rise to many different cell types, and since somatostatin and parvalbumin are not expressed in the MGE, we currently lack cell markers and genetic strategies to distinguish these cells. Ideally, we would like to determine the transcriptional profile of a cell undergoing fate decisions in the MGE while at the same time retain the ability to identify which type of interneuron (PV+ or Sst+) it becomes in the postnatal brain. To this end, Specific Aim 2 will create a spatially restricted and temporally inducible form of Dam-identification (DamID) to 'time-stamp' the transcriptome of MGE progenitors for future analysis. Initial experiments will utilize ESC technology that will lead to generation of new mouse strains to identify novel fate-determining genes in Sst+ and PV+ interneurons. Importantly, the proposed experiments should significantly advance our understanding of interneuron development and pave the way for Dr. Petros to secure an independent position in academia.